It happens in all the languages I'm learning. In lessons and in practice exercises, new words are introduced, sometimes with no hints. I make a guess (as others have suggested), but the problem with this is that sometimes I associate my guess with the new word rather than the correct translation and then it takes far longer to learn the word than if I saw the correct translation with the first instance. Ocassional my guess is correct and that feels good!

I disagree with Thomas Heiss that we should need to use another website to pre-learn words for Duolingo tasks. Often I struggle to find time to do one exercise here, having to work on 2 or more sites wouldn't work for me. Other sites are (I'm sure) very useful and I think the more sources one uses to learn, the better one will be, however, that's not the question being asked here.

I think we just have to accept that this is how Duolingo works - after all, when you speak to a native speaker they will use words you've not heard before (happened to me last night at a meeting in Welsh!) so it's good practice for real life!

sometimes I have to back up or start the lesson over again when I get that on a new lesson.

Personally I can remember intro "picture type" excercises when doing new lessons, where you have to select one of three pictures (multiple-choice).

The picture had the word written below and it's sound may have been played (or not).

I liked those excercises when I had seen a word the very first time....and for skills like food or animals, nature etc. this type made much more sense than only text type translations (with orange highlighting and hints).

What you described is the other picture type excercise for 2nd or 3rd time presentations of a single word (in a single learn new lesson session or strengthen review excercise).

Yes this happens to me all the time in norwegian!! one of the recent lessons (Health) started off with about six of those picture ones in a row, and I just typed in random letters cause I had no idea what they were!!

It happens to me all the time in the Swahili course. This is a totally foreign language to me and it's really hard to guess which word is the right one.... I was also wondering whether this is some kind of a bug. Sure, using the Tinycards would be useful, but I haven't seen them on the tablet app, which I'm mostly using + we are currently not getting any XP for them... (and it is motivating to see the XP going up..)

to learn from your mistakes

I agree with you. I have seen this in the Spanish course. Some comments state that "you learn from your mistake", a mistake costing me a health point which I can't afford to waste when learning a new subject.

In randomizing the questions, they sometimes ask about things they have not yet introduced.

Duolingo teaches largely through trial and error. You are supposed to make the best guess you can, mentally note if your answer was correct or not, and, ideally, do better next time (or the next or the time after that). Making mistakes is part of learning, not something to be avoided at all costs.

I find it best to do the tinycards for the lesson before doing the lessons on duolingo because otherwise I will keep being penalized and it's annoying. This only works on Spanish though, I don't think Japanese has tinycards.